Prevention of rusting of steel



3,520,738 PREVENTION OF RUSTING OF STEEL Hiroyoshi Matsubara and Yasnhiro Ishiyama, Yokohamashi, Japan, assignors to Nippon Kokan Kabushiki Kaisha, Tokyo, Japan N Drawing. Filed Jan. 17, 1967, Ser. No. 609,753 Claims priority, application Japan, Jan. 18, 1966,

Int. Cl. C23f 7/10 US. Cl. 1486.15 3 Claims ABSTRACT OF THE DISCLOSURE This invention relates to the prevention of rust on steel stock such as sheet stock, bar stock, etc., obtained. from the steel mill, by treating the surface of the steel stock with phosphoric acid after the steel is rolled so as to form a secure phosphate film on the surface of the steel which prevents the formation of scale and rust.

It is Well known that when steel stock is manufactured by rolling operations scale is formed and remains on the surface of the steel. It has been necessary to remove such scale for example by shot blasting, in order to improve the ability of the surface of the steel to accept coating materials which are applied for the purpose of protecting the surface. If the scale is not removed then rust inevitably forms, thus degrading the quality and appearance of the resulting products. It is therefore necessary to take special care in the packing, storage and handling of the steel stock.

It is accordingly a primary object of the present invention to provide a new method of treating steel stock in order to render the same rust proof.

It is another object of the present invention to provide in a simple and economical manner steel stock products having excellent corrosion resistance and the ability of accepting coating agents by preventing the formation of scale on the surface of the steel stock after the steel stock is obtained by the rolling operation.

It is yet a further object of the present invention to provide a method of protecting steel stock and the surface thereof so that no scale is formed thereon, whereby the steel stock can be further treated to obtain products therefrom without the need of scale removal by shot blasting or the like.

Other objects and advantages of the present invention will be apparent from a further reading of the specification and of the appended claims.

With the above and other objects in view, the present invention mainly comprises the treating of steel stock at a temperature of at least 400 C. with aqueous phosphoric acid so as to form a phosphate film which adheres to the surface of the steel and prevents the formation of scale thereon.

It has been found that it is possible to eliminate the above mentioned difficulties of the prior art in connection with the formation of rust on steel stock by simple means whereby a solution of phosphoric acid is applied to the surface of the steel stock during the manufacturing thereof in order to render the same rust proof and positively preventing the formation of scale thereon. This is accomplished according to the present invention most preferably by treating the steel stock immediately after the same passes through the rolls of the rolling mill at which time the steel stock is substantially free of scale. According to the present invention this condition of minimum scale on the surface of the rolled steel stock is utilized to the extent that the phosphoric acid solution is applied thereto at this time so as to most effectively form an adnited States Patent 0 3,520,738 Patented July 14, 1970 hering phosphate film on the surface which renders the same substantially completely rust proof.

The phosphoric acid solution which is utilized according to the invention should preferably have a concentration of about 0.5 to 20% by weight, and the phosphoric acid solution should be applied to the steel stock at a time when the temperature of the steel stock is at least about 400 C.

While some rust proofing can be achieved using phosphoric acid concentrations of less than about 0.5 the degree of rust proofing is not fully satisfactory so that it is advisable to use a concentration of at least 0.5% to achieve the fully satisfactory degree of rust proofing which is desired. At concentrations above about 20%, the rust proofing is effective, however, the viscosity of the solution becomes too high to insure uniform spraying and uniform distribution of the: phosphoric acid over the entire surface of the steel stock. It is most preferred that phosphoric acid of a concentration of about 1.0% to about 7.0% be used in order to provide satisfactory surface characteristics and economical rust proofing of the steel stock.

It has further been found that the temperature of the steel stock should be at least about 400 C. at the time that the phosphoric acid solution is applied thereto. Higher temperatures are still more preferred. Thus, a temperature of more than 700 C., which is the temperature of the steel stock at the time of completion of hot rolling, is highly advantageous from the standpoint of preventing rusting and forming corrosion resistant surface films. Even temperatures of 900 C. or higher are extremely desirable. At a temperature of about 400 C. scale still has not started to form on the hot rolled steel stock so that at this temperature it is still possible to apply the phosphoric acid solution to the surface of the steel stock in order to form a strongly adhering phosphate film which prevents the formation of rust. However, at temperatures beloW about 400 C. scale starts to form on the surface of the steel stock, and once this scale has formed it is impossible to form a satisfactory phosphate film on the stock.

Thus, in accordance with the present invention, by applying suitable phosphoric acid solution under proper conditions onto the surface of hot steel stock, a phosphate film is rapidly formed thereon which is effective to prevent formation of scale, thereby resulting in obtaining steel stock having excellent degree of corrosion resistance and coating properties, which exhibit a highly desirable appearance over a long period of time.

The following example is given to illustrate the present invention. The scope of the invention is not, however, meant to be limited to the specific details of the example.

EXAMPLE A solution of phosphoric acid of 5% concentration is sprayed onto the surface of a hot steel plate of a thickness of 11.6 mm., a width of 1600 mm. and a length of 19 meters, at the time that the steel plate emerges from the hot rolling mill, the temperature of the steel plate at this time being somewhat above 900 C. Although spraying is the most simple means of applying the phosphoric acid to the steel plate, other means of application can be used.

The thus applied phosphoric acid solution functions to instantly form a uniform, beautiful, strong phosphate film which closely adheres to the surface of the steel plate, which at the time of application of the phosphoric acid is still free of any scale due to the high temperature of the steel plate. The phosphate film thus formed functions to effectively protect the surface of the steel plate and to eliminate the need for later scale removal procedures such as by shot blasting, thereby resulting in a substantial cost reduction of the final product and further resulting in the production of a high quality product having an excellent degree of corrosion resistance and ability to be coated, as well as having a fine appearance over a long period of time.

For testing purposes the treated steel plate prepared in accordance with this example was left standing for one and one-half months side by side with an identical piece of hot rolled steel plate which was not treated with the phosphoric acid solution. A large quantity of red rust formed on the untreated steel plate, while the steel plates treated in accordance with the present invention exhibited no rust formation whatsover.

Thus, by applying a solution of phosphoric acid onto the surface of steel stock which is essentially free of any scale at the time of application, that is generally immediately following the hot rolling operation, a phosphate film is formed on the steel stock by the utilization of the heat of the steel stock toeffectively prevent oxidation and other undesirable phenomenon. Thus, it is possible in accordance wtih the present invention by simple means to avoid the difficulties encountered in previous methods and to provide products of high commercial value, of good appearance, being corrosion proof to an extremely high degree, and having the ability of being coated, all remaining for long periods of time. A further advantage of the present invention is the economy thereof due to the fact that the treating liquids utilized, namely phosphoric acid solutions, are readily available on the market and are substantially cheaper than known treating agents.

While the invention has been described in particular with respect to the formation of a phosphate film on hot rolled steel stock, it will be apparent that variations and modifications of the invention can be made withofifide parting from the spirit or scope of the invention. Accordingly, such variations and modifications are meant to be comprehended within the scope of the appended claims.

We claim:

1. Method of preventing corrosion of steel stock, which comprises applying a solution consisting essentially of aqueous phosphoric acid onto the surface of hot steel stock immediately after hot rolling operation for formation of the steel stock and prior to cooling to a temperature sufiiciently low for scale to form, so as to form a phosphate film on the steel stock, thereby protecting the surface of the steel stock and preventing subsequent formation of scale thereon.

2. Method according to claim 1 wherein the concentration of phosphoric acid is between about 0.5% and 20%.

3. Method according to claim 1 wherein the concentration of phosphoric acid is between about 1.0% and 7.0%.

References Cited UNITED STATES PATENTS 1,761,186 6/1930 Baker 148-615 2,813,813 11/1957 Ley et a1. 1486.15

FOREIGN PATENTS 383,116 12/1964 Switzerland.

RALPH S. KENDALL, Primary Examiner US. Cl. X.R. 

